Nearly two years to the day after Britney Spears released her memoir, the Princess of Pop's ex-husband Kevin Federline is set to publish his own tell-all with the fiery title You Thought You Knew. "Kevin spent about three or four years working on the book," a source exclusively tells Us Weekly. "This wasn't an overnight thing that he threw together after Britney came out with her book. He's never really told his side of the story in depth, and he knows interviews and documentaries can be twisted, so he wanted people to hear it straight from the horse's mouth.